Not applicable.
Not applicable.
This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for collecting data corresponding to every element in a CT array to generate a large number of separate slice images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurement, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
During data acquisition the table is translated through the gantry during gantry rotation such that a xe2x80x9chelicalxe2x80x9d data set is collected. An exemplary helical acquisition system is described in U.S. Pat. No. 5,974,110. As well known in the art table translation speed is related to the number of rows of data that can be collected during helical acquisition. To this end, table translation speed can be increased by increasing the number of rows of data collected.
A typical detector array includes a plurality of detector modules, each module forming a flat detector surface. The modules are positioned together so as to form an arc that is essentially centered on the X-ray source. Under each module surface a typical module includes rows and columns of detector elements aligned with X and Z coordinates, respectively. Detector elements are relatively small (e.g., 1.25 mm across). The modules are typically arranged side-by-side to form an array arc with module rows aligned so that data corresponding to a single array row can be used to generate a single thin slice image through a patient.
In addition to including the above described hardware a typical CT system also includes acquisition circuitry that acquires the intensity signals generated by the detector elements, converts the intensity signals into the CT counts and stores the CT counts as data for subsequent image reconstruction via back projection or the like.
There are many different CT applications and each application may ideally require data corresponding to either thin or thick image slices depending upon the application. For this reason many CT systems include switches that allow a user to cause acquisition channels to effectively sum the data from several adjacent array rows during acquisition. Thus, for example, to collect data for a patient slice that is twice as wide as the data corresponding to a single array row, the acquisition channels can be linked to the rows to collect projection data for two adjacent rows simultaneously. Similarly the acquisition channels can be linked to collect data from four adjacent rows simultaneously to generate an even thicker slice image. For exemplary prior art that teaches systems of this type see U.S. Pat. Nos. 5,291,402 and 5,982,846.
Recognizing the need to generate both thick and thin slice images to decrease detector module connector density and, in an attempt to minimize overall system costs, many CT system designers have opted to provide systems with fewer acquisition channels than detector elements. For example, an exemplary module may include 16 rows and 16 columns of elements for a total of 256 elements and the system may only include 128 acquisition channels to acquire the signals from the 256 elements (i.e., a 2 to 1 ratio of elements to acquisition channels). In this case, during data acquisition several different acquisition options are selectable including data corresponding to 1 through 8 separate thin slice images and data corresponding to 2 row thick slice images, 3 row thick slice images and 4 row thick slice images.
While more than 8 thin slice images are not routinely required in CT applications and therefore are not generally supported by CT hardware, there are certain applications where additional thin slice images are required. For instance, in the case of cardiac and CT angiography (CTA) procedures large numbers of thin slice images must be acquired quickly to facilitate efficient clinical practice. In these cases collection of 16 thin slice images instead of 8 is useful.
In addition, as indicated above, during helical data acquisition it is advantageous to collect data corresponding to as many rows as possible so that table translation speed can be maximized.
One solution to increase the number of rows of data collected is to use the system described above twice to collect data corresponding to 8 thin slice images each use for a total of 16 images. Unfortunately, not only does this option require additional time but it also may lead to collecting data corresponding to differing patient positions (i.e., a patient may move or breath, etc., between acquisitions).
Another solution is to provide additional acquisition circuitry so that there is a separate channel for every detector element in an array. This option, while possible, would increase overall system cost appreciably and therefore is not optimal.
An exemplary embodiment of the invention is to be used with a CT imaging system including less acquisition channels than detector elements such that separate data-sets for each element cannot be acquired simultaneously via the acquisition channels, the method for acquiring data corresponding to a number of elements that is greater than the number of acquisition channels during an acquisition period and comprising the steps of: for at least one acquisition channel, (a) during a first sub-period: acquiring a first data-set corresponding to a first detector element and storing the first data-set(b)during a second sub-period: acquiring a second data-set corresponding to a second detector element and storing the second data-set.
The above describe method enables collection of data corresponding to more elements than there are acquisition channels so that the acquired data can be used to construct many images corresponding to thin slices through an object being imaged. More specifically, where the elements are arranged in rows and columns to form an array and the array is mounted to a gantry for rotation about a rotation axis parallel to the columns so that data can be collected corresponding to several gantry positions, the method enables acquisition of data that can be used to generate a separate image for each one of the array rows or, in the case of helical imaging, to simultaneously collect data corresponding to a relatively greater number of rows so that table translation speed can be maximized.
In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to claims herein for interpreting the scope of the invention.